1) Field of the Invention
The present invention relates to a technology for fixing an image carried on a recording medium by being passed through a space between a fixing belt and a counter roller.
2) Description of the Related Art
It has been known in the conventional technology to mount a fixing device of such a type on an image forming apparatus, for example, a digital copier, a printer, a facsimile, and a multifunction machine provided with at least two functions of them (see Japanese Patent Application Laid-Open No. 11-2982. Preferably, the fixing device of this type, as well as fixing devices of other types, is configured to elevate a temperature on the fixing belt up to a value suitable for fixing an image in as short a warm-up time as possible. A longer warm-up time forces an operator to suffer a longer stand-by time. Various fixing devices with reduced warm-up times have been proposed in the conventional technology to solve such the problem, though there is a need for a further reduced time.
The conventional fixing device of this type, on the other hand, always supplies power at a constant ratio to the first heater for heating the fixing belt and the second heater for heating the counter member to operate them. The fixing device thus configured causes various problems. For example, this configuration causes a problem to extend the warm-up time for the fixing device.
It is an object of this invention to solve at least the problems in the conventional technology.
A fixing device according to one aspect of the present invention includes a fixing belt which is driven rotationally, a counter member disposed opposite to the fixing belt, a first heater which heats the fixing belt, and a second heater which heats the counter member. The fixing belt and the counter member define a nip therebetween, through which a recording medium carrying a toner image to be fixed thereon is passed such that the toner image is directed to contact a surface of the fixing belt, in the presence of heat and pressure applied to toner in the toner image during passage thereof to fix the toner image on the recording medium. A target temperature on the fixing belt during fixation and first and second reference temperatures are previously set, first and second reference temperatures being lower than the target temperature. A maximum allowable power is supplied to the first heater to heat the fixing belt when the temperature on the fixing belt is lower than the first reference temperature during activation of the fixing device. The fixing belt is driven rotationally, power supplied to the first heater is decreased, and power is also supplied to the second heater when the temperature on the fixing belt reaches the first reference temperature. Fixation is allowed to start at a point in time at which the temperature on the fixing belt reaches the target temperature and the temperature on the counter member reaches the second reference temperature.
A fixing device according to another aspect of this invention includes a driving roller, a plurality of belt supports including the driving roller, a first heater, an endless fixing belt wound around between the belt supports and heated by the first heater; and a second heater. The device also includes a counter member disposed opposite to one of the belt supports via the fixing belt and heated by the second heater; and a temperature detector which detects a surface temperature on the fixing belt. If the surface temperature on the fixing belt is lower than a predetermined temperature during activation of the device, a first power is supplied to the first heater. If the surface temperature on the fixing belt reaches the predetermined temperature, a second power is supplied to heat the second heater, at least the fixing belt is driven rotationally, and the power supplied to the first heater is dropped to an amount equal to an amount obtained by subtracting the second power from the first power.
A fixing device according to still another aspect of this invention includes at least two belt supports, and a fixing belt wound around between at least the two belt supports, a counter member disposed opposite to one of the belt supports via the fixing belt. The device also includes a first heater which heats the fixing belt, a second heater which heats the counter member, and a temperature detector which detects a temperature on the fixing belt. The fixing belt and the counter member define a nip therebetween, through which a recording medium is passed to fix an image on the recording medium. The device further includes a power controller which varies a ratio of power supplied to the second heater to power to the first heater based on the temperature detected by the temperature detector.
A fixing device according to still another aspect of this invention includes at least two belt supports, a fixing belt wound around between at least the two belt supports, and a counter member disposed opposite to one of the belt supports via the fixing belt. The device also includes a first heater which heats the fixing belt, and a second heater which heats the counter member. The fixing belt and the counter member define a nip therebetween, through which a recording medium is passed to fix an image on the recording medium. The device further includes a time detector which detects a time of supplying power to at least one of the first and the second heaters, and a power controller which varies a ratio of power supplied to the second heater to power to the first heater based on the time detected by the time detector.
An image forming apparatus according to still another aspect of this invention includes the fixing device according to the present invention.
The other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.